[S-CG55] Oceanic plate as inputs to subduction zone: from oceanic spreading ridge to subduction trench

[SCG55-08] Characteristics of normal faults developing in the outer rise of the Japan Trench

*YUE SUN¹, Ehsan Jamali Hondori¹, Jin-Oh Park¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Japan Trench, Outer rise, Multi-Channel Seismic (MCS), Normal fault, Petit spot volcano

There have been several large earthquakes occurred in the Japan Trench subduction zone and outer rise. Many previous studies have been conducted to elucidate the shallow crustal structure of the Japan Trench outer rise, but most of these studies depend on seafloor topographic maps and there are few studies focusing on the faults which cut the acoustic base in the Multi-Channel Seismic (MCS) survey profile. In order to investigate the characteristics of normal faults developing in the Japan Trench outer rise, the "Portable Multi-Channel Seismic Survey System" of the Atmosphere and Ocean Research Institute (AORI) of the University of Tokyo was mounted on the research vessel Shinsei-maru, for the survey KS-19-05 in April 2019 and survey KS-20-14 in September 2020, and a total number of 5 survey lines of MCS data were acquired. In this study, the data from three survey lines, KS19-05-1, KS19-05-5, and KS20-14-5 are used. Line KS19-05-5 of survey KS19-05 and line KS20-14-5 of survey KS20-14 generated strong noise due to bad sea conditions and poor equipment conditions. I succeeded in removing the noise by manually deleting the traces with spike noise and applying the Band-pass filter and FX deconvolution. Furthermore, the MCS data processing was performed with a Kirchhoff pre-stack depth migration (PSDM) and seismic depth sections were obtained. Three survey lines confirmed a clear Horst-Graben structure in the subducting Pacific plate. The results of investigating the normal faults, which are caused by plate bending and are imaged in the three survey lines, show that the normal faults of the survey line KS19-05-1 have a larger fault offset compared to the faults in the survey lines KS19-05-5 and KS20-14-5. However, the horizontal spatial distributions of these faults tend to be sparse. In addition, a number of young volcanic activities are observed near the trench axis in line KS19-05-1 and near the eastern side of line KS19-05-5 and line KS20-14-5. These young volcanic activities are referred to by Hirano et al. (2006) as petit spot volcanoes of site A and site C. In the PSDM section, petit spot volcanos and traces of magma intrusion were confirmed. In particular, on the line KS19-05-1, the reflection horizon of the chert layer is unclear within a range of 75 km from the trench axis to the seaside, and the existence of the chert layer cannot be confirmed. The sediment layer thickness was also clearly reduced in the petit spot volcanic area. I evaluated the formation time of the normal faults by calculating the difference between seafloor offset and the acoustic basement offset following the method of Ranero et al. (2003). In the line KS19-05-1, most of the normal faults are recent faults but in the line KS19-05-5 and the line KS20-14-5 both recent faults and older faults are seen on the seismic depth sections. Based on the above evaluations, the fault developing characteristics in the Japan Trench outer rise are organized into two models. In Model 1 the normal faults are not affected by the petit spot volcano, where older faults and recent faults are mixed and both are developing. In Model 2 the normal faults are affected by the petit spot volcano and only recent normal faults are developing. Model 1 applies to offshore Miyagi and model 2 applies to offshore Sanriku. The most important condition affecting the two models is the presence of the petit spot volcano.